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Frostbite? Not for These Microbes

With a splash of a particular chemical solution, microbes can survive in large numbers at temperatures as low as -80°C, researchers report. Because similar chemical cocktails exist on the cold surfaces of Mars, the Moon, and Europa, the findings may increase the chances of finding life on other worlds.

Scientists used to think that life couldn't survive in conditions that were either too hot or too cold. In the 1980s, however, biologists discovered microbial life forms, dubbed "extremophiles," which thrive in harsh environments. Bacteria that live near deep-sea hot-water vents, for example, can live at temperatures of up to 120°C. But researchers still believed that extreme cold was off-limits for microbial growth. That's because biological systems rely on liquid water to survive, and cell membranes become rigid when they get too cold.

However, a team of microbiologists led by John Hallsworth of Queen’s University Belfast hypothesized that chaotropic solutes, a type of solute that is known to disorder cellular macromolecules, would not only prevent water from freezing around the microbes but also could reverse the rigidifying effects of cold temperatures.

To test this, the researchers first established that the solute glycerol, which is already routinely used as a macromolecule protectant to preserve cells in the laboratory at low temperatures, becomes chaotropic at high concentrations. Then they monitored the growth of extremophiles on media that were supplemented with several chaotropic solutes and compared this with growth on media containing kosmotropic solutes, which have the opposite effect on macromolecules. In an experiment using four different types of cold-tolerant xerophilic fungi, a type of extremophile found in habitats with little water, the researchers showed that the fungal strains grew equally well on media supplemented with chaotropic or kosmotropic solutes at 30°C. However, at temperatures as low as 1.7°C, they recorded more growth on chaotrope-supplemented media than on media supplemented with kosmotropic solutes. There was no growth at all on some kosmotrope-supplemented media, the team reports online today in the Proceedings of the National Academy of Sciences.

Hallsworth and colleagues then exposed spores that were harvested from the xerophilic fungi to a nail-bitingly cold temperature of -80°C. They found that although 60% of the kosmotrope-treated fungal spores died, only 5% of those that were treated with a chaotropic solute died.

The find could have implications for discovering life on other planets, says Hallsworth. Scientists consider a world habitable if surface temperatures fall within the so-called Goldilocks Zone, between 0°C and 100°C. But many worlds that we know of contain the ingredients to make chaotropic solutes, he says, meaning that this zone may be "considerably more extensive than we could have previously envisaged.”

Astrobiologist Richard Hoover of NASA’s National Space Science and Technology Center in Huntsville, Alabama, agrees that the Goldilocks Zone needs to be expanded. But he says that the researchers shouldn't have limited themselves to looking at oxygen-breathing, or aerobic, microbes such as xerophilic fungi. Most extremophiles found in cold environments live without oxygen, he notes.